This depends on the material that the helmet is made of (also how the air compressor works--I would recommend a device designed for decreasing pressure, not increasing it), and how strong a vacuum is required.

There isn't any experimentally attainable perfect vacuum, so really what we are talking about here is reduced pressure. It is easiest to talk about this in terms of orders of magnitude: a household vacuum cleaner can get to about 20% of atmospheric pressure, and a cheap pump can probably achieve pressures about 1% of atmospheric pressure (10–2 atm). I routinely work with pumps that are about $5000, and can reduce pressure within a thick glass vessel to about 10–4 atm, and have occasionally used multiple pumps together to achieve pressures of 10–5 atm. I once worked in a lab that ran experiments in large steel boxes (>3 cm thick walls) under ultra high vacuum of about 10–10 atm, but that required about $50,000 worth of pumps, sensors and regulation systems. I think deep space can have pressures as low as 10–20 atm.

Given prior posts of yours I feel compelled to insist that you * DO NOT* try to put your head in anything that you have evacuated--even pressures achievable with a household vacuum cleaner can cause blood vessels to burst in your eyes, ears and face--I promise this is not pleasant!

If instead you wanted to make something akin to a thermos (an inner vessel and an outer vessel, with reduced pressure in between) that wouldn't pose as obvious a risk, but if you have ever broken a thermos you would understand why putting your head in such a contraption could still be very dangerous.

This depends on the material that the helmet is made of (also how the air compressor works--I would recommend a device designed for decreasing pressure, not increasing it), and how strong a vacuum is required.

There isn't any experimentally attainable perfect vacuum, so really what we are talking about here is reduced pressure. It is easiest to talk about this in terms of orders of magnitude: a household vacuum cleaner can get to about 20% of atmospheric pressure, and a cheap pump can probably achieve pressures about 1% of atmospheric pressure (10–2 atm). I routinely work with pumps that are about $5000, and can reduce pressure within a thick glass vessel to about 10–4 atm, and have occasionally used multiple pumps together to achieve pressures of 10–5 atm. I once worked in a lab that ran experiments in large steel boxes (>3 cm thick walls) under ultra high vacuum of about 10–10 atm, but that required about $50,000 worth of pumps, sensors and regulation systems. I think deep space can have pressures as low as 10–20 atm.

Given prior posts of yours I feel compelled to insist that you * DO NOT* try to put your head in anything that you have evacuated--even pressures achievable with a household vacuum cleaner can cause blood vessels to burst in your eyes, ears and face--I promise this is not pleasant!

If instead you wanted to make something akin to a thermos (an inner vessel and an outer vessel, with reduced pressure in between) that wouldn't pose as obvious a risk, but if you have ever broken a thermos you would understand why putting your head in such a contraption could still be very dangerous.

What material would you suggest I was thinking a motorcycle helmet which is has a duel layered shell to form a vacuum of sorts while riding. I was thinking to seal the air vents with sealer, and use a compressor to reduce pressure.

The thermal conductivity of a gas is almost independent of pressure until you get to very low pressures (lower that you will get with a gerry-rigged vacuum pump) so there's no realistic way you can do this.To keep that good a vacuum is very difficult and would need thick heavy walls made from glass or metal.

If you are thinking of using a vacuum as a shield against ultrasonic signals:

It is true that ultrasound cannot pass through a vacuum

However, the very rigid walls that are needed to contain the vacuum will conduct ultrasound around the vacuum fairly efficiently.

I think that the absorbent foam used inside a bicycle helmet would be a fairly good absorber of ultrasound, and a lot lighter than a vacuum helmet. Just don't use the air-cooled models with big gaps in them...

Seriously, you don't need to worry about ultrasound. But if you are worried, maybe a better protection is to get a dog? They can hear any ultrasound, and lead you away from it.

What about a dual layered titanium steel helmet with foam inside? I'm assuming it wouldn't be a perfect vacuum but you could get the pressure lower then a thermus without fear of ot crushing your head?

Foam is not a vacuum. You can indeed evacuate a double-wall steel shell but the military find modern composites give better protection and lighter weight.

If you know the frequency of the alien ultrasound (use a bat detector to scan it) you can make "ultrasound chaff" with a car reversing transducer and baffle the buggers. 40 kHz travels reasonably well through air, and anything much higher than 1 MHz will be completely deflected and defocused at an air/Kevlar or air/steel interface, so grandad's WWII trophy will be perfectly adequate.

This depends on the material that the helmet is made of (also how the air compressor works--I would recommend a device designed for decreasing pressure, not increasing it), and how strong a vacuum is required.

There isn't any experimentally attainable perfect vacuum, so really what we are talking about here is reduced pressure. It is easiest to talk about this in terms of orders of magnitude: a household vacuum cleaner can get to about 20% of atmospheric pressure, and a cheap pump can probably achieve pressures about 1% of atmospheric pressure (10–2 atm). I routinely work with pumps that are about $5000, and can reduce pressure within a thick glass vessel to about 10–4 atm, and have occasionally used multiple pumps together to achieve pressures of 10–5 atm. I once worked in a lab that ran experiments in large steel boxes (>3 cm thick walls) under ultra high vacuum of about 10–10 atm, but that required about $50,000 worth of pumps, sensors and regulation systems. I think deep space can have pressures as low as 10–20 atm.

Given prior posts of yours I feel compelled to insist that you * DO NOT* try to put your head in anything that you have evacuated--even pressures achievable with a household vacuum cleaner can cause blood vessels to burst in your eyes, ears and face--I promise this is not pleasant!

If instead you wanted to make something akin to a thermos (an inner vessel and an outer vessel, with reduced pressure in between) that wouldn't pose as obvious a risk, but if you have ever broken a thermos you would understand why putting your head in such a contraption could still be very dangerous.

Do Igloo Coolers use reduced pressure as insulation? I think they do but probably not comparable to a steel dewar flask?

Do Igloo Coolers use reduced pressure as insulation? I think they do but probably not comparable to a steel dewar flask?

you still need to think about what Evan_au said re conduction through metal skin of a flask. Also think about weight.I think a motorcycle helmet would reduce the level enough, I really wouldnt go to the trouble of a vacuum.what will you do when you want to go to the bank? They wont let anyone in with a helmet!

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